1. Field of the Invention
The present invention relates to a process for the production in continuous of phthalodinitrile. More particularly the present invention relates to a process for the production in continuous of isophthalodinitrile or terephthalodinitrile by simultaneous amidation and dehydration of the respective acid chlorides in steam phase on a fixed bed of a dehydration catalyst.
2. Prior Art
Dinitrils of arylic acids can be prepared only by complex chemical methods or adopting sophisticated and expensive equipment. The classical method, from carboxylic acids by reaction with ammonia in steam phase of dehydration catalysts, is not easily applicable due to the chemical-physical characteristics of the starting acids. In fact many solid aromatic acids, such as, for example, phtalic acids, are difficult to vaporize, melt at high temperatures and already at these temperatures they begin to decompose developing carbon dioxide.
Consequently, attempts have been made, to overcome these difficulties in different ways, for example dispersing the pulverized aromatic acid in a stream of inert gas at a temperature below its melting point, mixing it with a very hot ammonia stream and conveying the vaporized reaction products on a dehydration bed. This method, claimed in U.S. Pat. No. 3,070,621, which avoids the melting of the aromatic acid, considerably reduces the decomposition phenomenon and therefore the forming of deposits and crusts on the reactor walls, especially in that zone used as evaporizer. Anyway it is evident that this results in considerable installation difficulties and it is therefore clear why successively other ways were tried, such as for instance the use of phtalic acids derivatives, which vaporize more easily: for example, ammonium salts, diamides and above all esters of such acids, wherein the alkyl chain contains from 1 to 4 carbon atoms. Thus, for example, DTAS No. 1,279,020 claims the preparation of aromatic nitriles from the methyl esters of the correspondent acids: these processes, if on the one hand, simplify the installations, on the other hand raise the new problem of the precursor production, which precursor are not easily available on commercial and industrial scale.
Only in the last 20 years new processes of ammono-oxidation were industrially developed, which consist in reacting aromatic hydrocarbon mixtures containing an alkyl chin, oxygen and air, and ammonia at high temperatures on suitable vanadium, tungsten catalysts, etc. However, these processes require high technologies, suited to effect the reactions in steam phase, on fluidized beds; separation and recycle of the unreacted compounds or of the reaction intermedies, use of particular and sophisticated catalysts, and in addition plants of considerably sizes are needed, which require high capital investments.
The Applicant has now surprisingly found a continuous process which allows to obtain isophthalodinitrile and terephthalodinitrile by adopting a simple technology, in plants of reduced costs and dimensions, starting from chlorides of the respective acids; i.e. from cheap row materials, commercially available and/or easily manufactured on industrial scale, and which finally allows a flexible production.